Planar, area-light emitting diode devices, both organic and inorganic are a promising technology for flat-panel displays and area illumination lamps. The technology relies upon thin-film layers of materials coated over an area upon a substrate. Such LED devices can include, for example, organic light-emitting diode (OLED) materials such as are described in U.S. Pat. No. 4,769,292, to Tang et al., U.S. Pat. No. 5,061,569, to VanSlyke et al., or quantum dots in a polycrystalline semiconductor matrix such as are described in US Publication 2007/0057263 by Kahen.
However, as is well known, much of the light output from the light-emissive layer in an LED is absorbed within the device. Because the light emission from thin-film LEDs tends to be Lambertian, light is emitted equally in all directions so that some of the light is emitted directly from the device, some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally, trapped, and ultimately absorbed by the various high-optical-index layers within the device. In general, up to 80% of the light can be lost in this manner.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light-emitting devices. For example, diffraction gratings have been proposed to control the attributes of light emission from thin polymer films by inducing Bragg scattering of light that is guided laterally through the emissive layers; see “Modification of polymer light emission by lateral microstructure” by Safonov et al., Synthetic Metals 116, 2001, pp. 145-148; and “Bragg scattering from periodically microstructured light emitting diodes” by Lupton et al., Applied Physics Letters, Vol. 77, No. 21, Nov. 20, 2000, pp. 3340-3342. Brightness-enhancement films having diffractive properties and surface and volume diffusers are described in WO 0237568 A1 by Chou et al. The use of micro-cavities and scattering techniques is also known; for example, see “Sharply directed emission in organic electroluminescent diodes with an optical-microcavity structure” by Tsutsui et al., Applied Physics Letters 65, No. 15, Oct. 10, 1994, pp. 1868-1870 and commonly-assigned U.S. Patent Application Publication No. 2006/0186802 by Cok et al. However, these approaches are limited in the amount of trapped light extracted, tend to create an unacceptable angular dependency on luminance or color, or disturb the polarization of incident light rendering circular polarizers ineffective, thereby compromising the performance of thin-film LED displays.
Reflective structures surrounding a light-emitting area or pixel are described in U.S. Pat. No. 5,834,893 to Bulovic et al. and describes the use of angled or slanted reflective walls at the edge of each pixel. Similarly, Forrest et al. describe pixels with slanted walls in U.S. Pat. No. 6,091,195. These approaches use reflectors located at the edges of the light-emitting areas. However, considerable light is still lost through absorption of the light as it travels laterally through the layers parallel to the substrate within a single pixel or light-emitting area.
U.S. Pat. No. 6,831,407 by Cok describes the use of a single type of topographical feature in a light-emitting device. However, the design described therein is also subject to light emission having a frequency and angular dependence that can be problematic for broadband emitters employed, for example, in devices employing a single, white-light emitting layer.